The present invention relates to a coaxial resonator of the type having an outer and inner conductor with the latter being concentrically disposed in the former in a short circuit relationship at one end thereof and in an open circuit relationship at the other end thereof wherein a dielectric member is disposed in the open circuit between the outer and inner conductors. The invention is useful for making a bandpass filter with a plurality of such coaxial resonators without altering the principal structure thereof.
Conventional quarter-wavelength coaxial resonators comprise an outer conductor having a closed end and an open end, and an inner conductor concentrically extending from the closed end of the outer conductor to the open end thereof. For purposes of making the coaxial resonator compact and making it have a high Q value, a dielectric member is provided in the open circuit between the outer and inner conductors. For frequency adjustment an electrode is connected to the open circuit end of the inner conductor in opposed relationship with a second electrode adjustably secured to a conductive housing. However, when the resonator is coupled to an external circuit the resonant frequency of the resonator tends to vary significantly depending on the coupling coefficient to such a degree that the frequency adjustment by the pair of electrodes is unable to compensate for such variation to obtain a desired value for the resonant frequency. This required the use of coaxial resonators which are tailored to meet the specific requirements of particular external circuits.